Kodaikanal, home to Indian’s oldest solar observatory, is known for its wide variety of flowers. Peruvian lily, cactus aloe, viola daisy, orchids, jasmine and chrysanthemum are some of the common flowers that greet visitors to Kodaikanal. Not so familiar are some 50 varieties of a rare species called Kurinji, of which a violet-coloured flower (Phelobophyllum Kunthanum) is famous, as it blooms only once in 12 years, exhibiting some strange, as yet unexplained, relation with the Sunspot cycle.
The flower is native to south India, seen in the Pazhani, Aanamalai and Nilagiri hills. Nowhere else in the world can one find this flower. It last appeared in 1994 and 12 years later, it was seen not only in Kodaikanal but also in the Nilagiri mountains and some tea gardens in parts of Kerala. Nature’s colorful signature was somewhat subdued, perhaps indicating a delayed start of the 24th solar cycle, now expected to peak in 2011.
It is possible to observe the Sun daily on the Internet (berkeley.edu). Everyday, the image of the sun is projected on line. In the white light image of the Sun, one can see black dots depending on the solar cycle. They are Sunspots, a mysterious solar feature, which has fascinated people down the ages.
Chinese observers had in fact noted them for at least one thousand years before the invention of the telescope. The Sun was one of the first celestial objects to be examined with the telescope. Even today, the Sun continues to attract special attention. The Americans set up a solar telescope in 2004 mainly to study the next two solar cycles.
Galileo was the first to notice Sunspots with his new instrument. He saw them in 1611 and 1612. He, however, hesitated to speak about them in public. For, a ‘defect’ in a celestial body was unacceptable to the Church and was contrary to the religious belief of the times. But the spots continued to kindle people’s curiosity. It is reported that Johannes Fabricus (1587-1616) of Holland was an early observer of Sunspots using a telescope.
It is somewhat strange to find that over two centuries had to pass by, before people thought of the Sunspots in terms of cycles appearing periodically. In 1843, Heinrich Schwabe pointed out, after 17 years of observation of the Sun that the number of Sunspots seemed to rise and fall in regular cycles. He found a ten-year interval between two peaks of Sunspot activity. The length could vary from seven to 17 years.
If people were a little slow to recognize the cyclic nature of the spots, the reason was the Sun itself. In the 30 years following the discovery of Sunspots, there were only two periods of maximum spots, 15 years apart. Then the spots declined and disappeared almost completely. They reappeared only in 1715, after an absence of 70 years. Surprisingly, there was hardly any Sunspot between 1645 and 1715, which is called the Maunder Minimum. In 1715, when the now familiar curtains of light in the sky appeared over the polar regions, they made news in Northern Europe.
Similarly, there was low Sunspot activity between 1450 and 1550 (Sporer Minimum) and again during 1870-1890 (Dalton Minimum). Later, the maximum number of Sunspots was seen during 1989-1990 and the next maximum was in 2001. Naturally, people did not know whether a 34-year phenomenon or its 70-year absence was the normal occurrence. The Sunspot cycle has since been appearing. Changes in the surface activity appear to undergo an 11-year cycle.
What then are the Sunspots? In 1801, an English astronomer William Herschel thought they were holes to cooler places that might be inhabited! Though nothing could go near the Sun without being burnt out, Sunspot areas are indeed relatively cooler than the rest of the solar surface. Their temperature has been found to be 4300º K as against 6000º K on the surrounding white surface. That is why they appear as black dots. The dark centre in the dots is called Umbra, which is surrounded by a lighter rim known as Penumbra.
Sunspots are shallow depressions, below a few hundred kilometers of the surface of the Sun. Many of them are round. But some are oblong too. They change their shape as they travel around the Sun. An average spot is about the size of the Earth! The largest could gobble up Jupiter! Their number varies from a pair to several hundreds. They wax and wane with the solar cycle. They range from six at a time near a solar minimum to over a hundred near a solar maximum. But the Sun is so big that even the largest number of spots could cover just one per cent of the solar surface.
Frequency of Sunspot: A Puzzle
The frequency of the spots is governed by the solar cycle. The earliest spots in a cycle are found around latitudes 40º north and south of the solar equator. As the cycle goes on, they come closer to the equator but stop within 10º of it. It has been found that the first spot after a solar minimum, invariable appears between latitudes of 30º and 40º on either side of the equator. They migrate towards the equator over 4½ years and decline over the next 6½ years of cycle. It is remarkable to note that even as the last spots disappear, the first spots of a new cycle come on the scene at a distance. In 1922, a British pioneer in solar physics, W.E. Maunder (1851-1928) drew the attention of the observers to the change in the latitudes of the Sunspots.
Sunspots are dark because they are sites of strong magnetic fields. These fields prevent the transport of energy from the solar interior via convection, while the photosphere—the visible surface in white light—receives the energy from below. The magnetic fields spread out with height and become more dominant at higher layers. They cause the non-radiative heating of the upper atmosphere, called the chromosphere. The magnetic fields make the chromosphere appear highly structured in the form of many magnetic features.
This can be seen in any picture of the Sun taken in the hydrogen line at 656.3 nm. One of the interesting observations has recorded that the temperature above the Sunspots shoots up at the chromospheric level. The bright regions that are seen in both hydrogen alpha and calcium lines are called Plages. The plages become further intensely bright during solar flares. Another interesting magnetic feature that we see in the choromsophere in the H-alpha line is the dark thread-like ‘filaments’. As the Sun rotates, the same dark filaments come close to the limb and appear as ‘prominences’. As there is no background intensity of light from the photo-sphere near the limb the ‘prominence’s’ appear as bright magnetic loops.
The filaments are dense and cool material suspended in the hot chromosphere. What supports the dense material in the filament against gravity of the Sun and from the surrounding hot temperature prevailing in the chromosphere? Also, how can the temperature in the choromosphere and in the corona (outermost layer of the Sun) can shoot up, while in the photosphere below, it is relatively low. The temperature is around 6000 K in the photosphere, whereas in the corona, it touches a million degrees or more. Astronomers search for a clue to these questions in the Sun’s magnetic field. The sun seems to have a magnetic cycle that lasts 22 years, unlike the 11-year Sunspot cycle.
Astronomer Hale observed that the Sunspots showed structures that seemed to follow magnetic lines of force. When the spectroscope was focused on a Sunspot, it appeared much stronger magnetically than the other regions of the Sun. The spectroscope also reveals that the Sunspot gases move from the center to the rim. The spots behave like a leader and a follower. Magnetic lines of force seem to come out from the ‘leader’ to the ‘follower’. Often, a pair of spots appears to form the north and south poles of a magnetic field. In other words, the pairs generally have opposite magnetic polarity. From one 11-year Sunspot cycle to the next, a total reversal of the Sunspot polarity occurs.
An instrument for recording the magnetic fields on the Sun was developed in 1951 by an American astrophysicist, Harold Delos Babcock (1882-1968) and his son, Horace welcome Babcock. The magnetograph operates by measuring the effect of magnetic fields on spectral lines. The spectral lines broaden due to the influence of magnetic fields, known as the Zeeman effect, named after its discoverer, Pieter Zeeman. The polarization of light resulting from the Zeeman effect is measure by polarimeters. In fact, the magnetic fields of the Sun are constantly changing. They precede a Sunspot and linger on long after it disappears. The spots are the outward symbols of the magnetic fields and help observers locate and measure the strength of the magnetic fields.
Over the years, observers have noted tell-tale signs indicating the birth of Sunspots. The first indication is increased brightness in the lower chromosphere. A little later, pores are seen on the photosphere.
After this, a group of spots appears. The leading spot is in the direction of Sun’s rotation. Indeed, the spots indicate that the rotation of the Sun, as generally observed, is different at different latitudes. The magnetic lines of force in a large group give rise to flares, as they reconnect themselves. Sunspots for all their prominent energy start to fade after ten days. Over a month they shrink. But the magnetic field stays on till the spot almost disappears. Sunspots are visible indications of intense solar activity. They do not radially point towards the Earth, except during the last years of the Sunspot cycle. This is because of the fact that the Earth moves around the Sun in an orbit inclined at about 7ºangle to the solar equatorial plane. Recent studies of the Sunspot cycle seem to indicate that the Sunspots, flares, prominences and other surface phenomena are just symptoms of more dynamic processes inside the Sun.
Closer observations revealed a link between Sunspots and solar flares. In a flare, the material near a Sunspot can explode into outer space. Studies in Kodai point out that the motions of Sunspots twist the magnetic fields, which then release the stored energy. A notable group of astronomers of the Indian Institute of Astrophysics (IIA) observed that the rotation of the Sunspots is different from their neighbouring surface rotation. As analysis of different Sunspots in the records at Kodaikanal indicates that the rotation rate of the Sun’s interior is different from that outside.
Unlike the Earth’s west to east rotation, Sunspots move from east to west. The rotation of the Sun is fastest at its equator. The accuracy observed of the rotation has considerably improved in recent years. The IIA team has measured the rotation to the highest accuracy viz. one metre per second. The team has found several characteristics of the solar rotation that would be of interest to the emerging study of helioseismology. The differential rotation of the Sun is believed to be the cause of the solar cycle.
In 1956 Robert Leighton (1919- 1997), an American professor, who made telescopes that covered the micrometer to millimeter region of the spectrum, also designed cameras in investigate the magnetic velocity fields of the Sun. He mapped the complicated patterns of the fields with excellent resolution. He discovered surface velocities on the Sun, which are today known as five-minute oscillations. Solar oscillations have been subsequently recognized as trapped acoustic waves, opening up a new field viz. helioseismology.
Another interesting feature of the Sun is its outermost ‘atmosphere’, called the corona (Latin for crown). A corona is best seen during a solar eclipse, as the Moon passes between the Earth and the Sun. The corona, a pearly halo, begins at about 16,000 km above the visible surface of the Sun and extends all the way to the Earth and beyond.
Let us look at some physics of the corona. If a substance is heated enough, it may end up as a charged atom with one or more electrons missing. An ion can be identified by its unique spectral lines. Spectroscopic measurements have shown that the surface temperature about a few million degrees hot is revealed by the presence of iron atoms (Fe 13+) without 13 of their electrons, removed by the extreme hear energy.
The corona appears differently at different times. The structure varies with the solar cycle. At the solar minimum, when the Sunspot activity on the surface of the Sun is limited to perhaps its equator, the corona exhibits bright streamers, mostly from the active regions around the equator. In contrast, the corona is compact during a solar maximum.
The fist 3D images of the Sun have been returned by two NASA satellites, STEREO-A and –B, launched in 2006 in orbits around the Sun, separated by only 4 degrees The idea is to image coronal mass ejections (CME) all the way from the Sun to the Earth. The images combine four wavelengths corresponding to 60,000 to 2.5 million degrees C in the corona. The two satellites get separated by about one degree a week and eventually provide in situ measurements of the coronal mass ejections striking the Earth’s magnetic field.